1. Field of the Invention
The invention relates to detecting an object on a support structure.
2. Brief Description of Related Art
Detection assemblies are particularly advantageously provided in a lithographic projection apparatus. Aspects of the present invention shall therefore hereinafter be illustrated by a lithographic apparatus by way of example only. In a lithographic projection apparatus, the presence of an object on a support structure is currently detected by measuring the capacitance of an assembly comprising at least cables connected to the support structure and the support structure itself. It is impossible to measure the capacitance of the support structure only, because the measurement of the capacitance of the support structure is carried out via the cables and is thus influenced by the cables and possibly by the presence of other components in electrical contact with the support structure. The cables, in particular, introduce a relatively high additional capacitance in this measurement. The cables are, however, needed to energize electrodes which are used to exert a suitable electrostatic clamping force on the object. The presence of additional capacitances makes it difficult to establish accurately the relatively small capacitance of the support structure which furthermore varies in accordance with the position of the object. It is advantageous to be able to measure the capacitance of the support structure accurately, among others, for the following reasons. This capacitance: (1) indicates whether the object has been correctly placed and/or clamped on the support structure, (2) gives, after calculation via the formula below, the maximum force which may be exerted on the object without the object becoming detached from the support structure, and (3) enables to establish with a high degree of reliability the presence or absence of an object on the support structure without the need of an additional calibration.
Clearly, it is presently impossible to measure accurately the relatively small variable capacitance of the support structure in a lithographic projection apparatus.